Cream the softened butter, brown sugar, and white sugar together.
Add the mashed banana and mix until combined.
Add the eggs and vanilla paste, then mix again. The butter may look split or curdled—don’t worry.
In a separate bowl, whisk together the flour, baking soda, and salt.
Combine the dry ingredients with the wet ingredients.
Add the buttermilk half at a time, mixing thoroughly after each addition.
Pour the batter into a lightly sprayed loaf pan lined with parchment paper.
Bake at 325°F (163°C) until deeply golden and firm to the touch, with an internal temperature of 200–205°F.
Immediately after removing from the oven, pour the maple syrup evenly over the top. It will absorb quickly, so pour in small sections and spread with a pastry brush if needed.
Sprinkle with flaky sea salt.
Allow to cool completely before slicing.